1. Field of the Invention
The invention relates to an image pickup apparatus such as a video camera having recording means or the like, in which the position of a photographing lens on the optical axis is moved by a stepping motor.
2. Related Background Art
In a recent image pickup apparatus (hereinafter, referred to as a video camera) such as a video camera or the like, in order to reduce the size and weight, in place of a conventional zoom lens in which a magnification changing lens and a focusing lens are interlockingly actuated by a cam ring, there is used a zoom lens of what is called an electronic cam system in which the cam ring is omitted and the magnification changing lens and the focusing lens are respectively driven by independent motors so as to keep a predetermined relation.
Nowadays, in the video camera having such a zoom lens, generally, the magnification changing lens is driven by a DC motor and the focusing lens is driven by a stepping motor.
However, since the vibration and noises of the stepping motor are larger than those of the DC motor, in the conventional video camera using the stepping motor, there is a problem such that the driving sound of the lens by the stepping motor is recorded as noises onto the video tape through a microphone of the video camera.
One technical means for solving the drawbacks as mentioned above is disclosed in Japanese Patent Publication No. 52-24649. The above technical means relates to what is called a microstep driving in which by controlling the power supply to a stepping motor, a rotor can be stopped at a pitch which is finer than the stop position of the rotor that is determined by the number of phases and the number of magnetizations of the rotor. Since the current supply to the coil almost smoothly changes even during the rotation, the occurrence of the harmful vibration and noises can be prevented.
However, when the lens is driven by the stepping motor using the current supplying method as mentioned above, an output torque of the stepping motor decreases as compared with that in the case where a rectangular wave shaped current is supplied. Therefore, the stepping motor of a larger electric power or a large shape is needed. There are problems such that a battery as a power source of a video camera is rapidly consumed and the realization of the small size and light weight of the video camera is obstructed.
As a technique to prevent the consumption of the battery and to drive the lens by a small motor, a combination with the means disclosed in Japanese Patent Publication No. 53-25085 is also considered. According to such a combination technique, the current supply waveform is changed in accordance with whether the stepping motor is driven at a high speed or a low speed. According to such a method, however, when the stepping motor is driven at a high speed, the current is supplied by a waveform which is closer to the rectangular wave. Consequently, there is a problem such that the occurrence of the vibration and noises cannot be prevented in the high-speed driving operation in which the vibration and noises are largest.